NoSQL is a better fit for Gmail because you wouldn't want one gigantic 'emails' or 'contacts' table, which is how you would model it in SQL. Per gmail account,

NoSQL is a better fit for Gmail because you wouldn't want one gigantic 'emails' or 'contacts' table, which is how you would model it in SQL. Per gmail account, you'd still dump all the mails into one gigantic emails table, either an SQL or a NoSQL one. As for the totality of the gmail accounts, well, you'd use sharding, which is the same whether you use NoSQL or SQL. You also want to cache the hell out of it, which is easier done in a NoSQL model (e.g. if the cache layer is integrated and hence uses the same API as the persistent stuff). But a SQL DB can also use the same transparent API for the persistent stuff with their standard Query API. It wouldn't even need to parse the SQL statement, just hash it and compare it. And you could also use a layer that makes it transparent, such as an ORM.